Geometric frustration in small colloidal clusters

TL;DR

This study uses simulations to explore how competing short-range attraction and long-range repulsion influence the structure and yield of small colloidal clusters, revealing the role of geometric frustration in nonergodic regimes.

Contribution

It demonstrates how geometric frustration affects cluster formation and yield in colloidal systems with competing interactions, especially in nonergodic states.

Findings

High yield of maximally bonded clusters at sufficient attraction strength for small sizes.

Geometric frustration reduces cluster yield in larger clusters (m≥7) in nonergodic regimes.

Special case of m=6 shows competition between different structures influenced by entropy and kinetics.

Abstract

We study the structure of clusters in a model colloidal system with competing interactions using Brownian dynamics simulations. A short-ranged attraction drives clustering, while a weak, long-ranged repulsion is used to model electrostatic charging in experimental systems. The former is treated with a short-ranged Morse attractive interaction, the latter with a repulsive Yukawa interaction. We consider the yield of clusters of specific structure as a function of the strength of the interactions, for clusters with m=3,4,5,6,7,10 and 13 colloids. At sufficient strengths of the attractive interaction (around 10 kT), the average bond lifetime approaches the simulation timescale and the system becomes nonergodic. For small clusters m<=5 where geometric frustration is not relevant, despite nonergodicity, for sufficient strengths of the attractive interaction the yield of clusters which maximise the number of bonds approaches 100%. However for $m=7$ and higher, in the nonergodic regime we find a lower yield of these structures where we argue geometric frustration plays a significant role. $m=6$ is a special case, where two structures, of octahedral and C2v symmetry compete, with the latter being favoured by entropic contributions in the ergodic regime and by kinetic trapping in the nonergodic regime. We believe that our results should be valid as far as the one-component description of the interaction potential is valid. A system with competing electrostatic repulsions and van der Waals attractions may be such an example. However, in some cases, the one-component description of the interaction potential may not be appropriate.